A Simple Microsoft 365 Security Hardening Checklist

Microsoft 365 is often one of the most important systems in a small business.

It can hold email, files, Teams messages, calendars, user accounts, devices, and admin access.

That makes it a major security target.

If an attacker gets into a Microsoft 365 account, they may be able to read email, steal files, reset passwords, create inbox rules, or access other systems.

This checklist covers the main areas I would review when improving Microsoft 365 security.

1. Turn On MFA

Multi-factor authentication, or MFA, should be one of the first checks.

Passwords get reused, guessed, leaked, and phished. MFA adds another layer of protection.

Things to check:

• Are all users covered by MFA?
• Are admin accounts protected by MFA?
• Are old authentication methods still allowed?
• Are users only relying on passwords?

MFA does not fix everything, but it reduces a common risk.

2. Review Admin Accounts

Admin accounts can make major changes.

That means they need extra care.

Things to check:

• How many global admins exist?
• Does each admin account belong to one person?
• Are shared admin accounts being used?
• Are admin accounts used for normal email and browsing?
• Are old admin accounts still active?

Admin access should be limited.

A user should not have admin rights unless they need them.

3. Use Least Privilege

Least privilege means users only get the access they need.

This lowers risk if an account gets compromised.

Review access to:

• Admin roles
• Shared mailboxes
• Microsoft Teams
• SharePoint sites
• OneDrive files
• Security settings
• Groups
• Third-party apps

The question should be simple:

Does this user or app need this access?

If the answer is no, remove it.

4. Clean Up Old Accounts

Old accounts are a common risk.

A former staff account should not stay active after the person leaves.

Things to check:

• Are old users still enabled?
• Are unused accounts still licensed?
• Are former staff accounts blocked?
• Has mailbox access been reviewed?
• Has file ownership been handled?
• Are group memberships still correct?

A good offboarding process protects the business.

5. Check Audit Logs

Audit logs help during an investigation.

They can show:

• Who signed in
• Where the sign-in came from
• What device was used
• What files were accessed
• What admin changes were made
• Whether mailbox rules were created
• Whether permissions changed

Logs do not stop attacks by themselves.

But without logs, it is much harder to understand what happened.

6. Review Device Security

Microsoft 365 security is not only about accounts.

Devices matter too.

A compromised laptop can put business data at risk.

Things to review:

• Are devices patched?
• Is antivirus active?
• Are devices encrypted?
• Are users local admins?
• Are lost devices removed?
• Are unmanaged devices accessing business data?

Identity security and device security should work together.

7. Reduce Email Risk

Email is a common entry point for attacks.

A review should include:

• Phishing protection
• External sender warnings
• Mailbox forwarding
• Suspicious inbox rules
• Attachment handling
• Link protection
• User training

A compromised mailbox can cause a lot of damage.

Attackers can read messages, send fake invoices, reset passwords, and target other staff.

Example Risk Table

Conclusion

Microsoft 365 security does not need to start with complex tools.

The basics matter first.

Turn on MFA. Reduce admin access. Remove old accounts. Review logs. Protect devices. Check email rules.

These steps lower risk and make the environment easier to manage.

Good security starts with clear controls that people can understand and maintain.